1. Field of the Invention
The invention relates to a method for measuring parameters relating to stability of joints in the human body. More specifically, the invention relates to such a method which includes the step of soft tissue compensation. The inventive method also includes the step of digitization, that is, the step of locating in three-dimensional space and in an appropriate coordinate system, the position of parts of the human body related to the joint. The method also includes the step of presenting data relating to motion and force in the appropriate coordinate system.
2. Description of Prior Art
In order to determine the stability of a human joint, it is necessary to gather data of the motion of the relative portion of the joint (that portion of the joint farthest removed from the body) relative to the reference portion of the joint (that portion of the joint closest to the body), with different forces applied to the relative portion. For example, in determining the stability of the knee joint, tibio/femoral motion (motion of the tibia relative to the femur) must be observed, with the application of different, known, forces to the tibia.
With present methods, as discussed in our U.S. Pat. No. 549,555, an examiner, e.g. a physician or a physiotherapist subjectively observes the motion so that comparison with the state of the joint between observations separated in time is a function of the memory of the examiner and the accuracy of his description viz-a-viz his observations. Furthermore, objective and accurate knowledge of the magnitude of applied forces is not available to the examiner.
It is apparent that a set of reproducible, objective measurements, would be superior both for recording a present condition and for comparison purposes.
It is also known that the application of force to the relative portion of a joint will cause motion of the reference portion thereof due to the soft tissue surrounding the reference portion. Present methods measure the total motion of the relative portions plus the reference portions due to applied forces, without consideration of the motion of the reference portion. As the critical data which is necessary for determining the stability of a joint is the motion of the relative portion relative to the motion of the reference portion thereof, such measurements are insufficient.
For example, and in considering specifically the knee joint, the ability to measure complete tibio/femoral motion is limited by the existence of soft tissue surrounding the femur. Thus, when force is applied to the tibia to cause motion thereof, force is also inevitably applied to the femur. This force will cause motion of the femur in the soft tissue surrounding the femur. A surface measuring device, which measures only the motion of the tibia, will therefore not be measuring the motion of the tibia relative to the femur which is also moving.
A technically ideal method of providing accurate bone versus bone measurement is to attach the measuring instruments by bolt screws to the bones. This is clearly unacceptable from a clinical point of view.
A further problem of measurement systems presently available is that they present data in the coordinate system of the measuring instrument rather than in the coordinate system of the relative portion of the joint. Thus, the examiner must attempt to visualize what is going on in the coordinate system of the relative portion of the joint from data which is displayed in the coordinate system of the measuring instrument.
Referring once again to the knee joint, the measurement of total motion of bones in this joint requires accuracy on the order of one millimeter and one degree. The total motion (three translations and three rotations) is ideally required because of the complex sliding and rotational motions occurring in the knee. These motions must be measurable at all desired flexion angles. Measurement devices available range from accurate unidirectional transducers to much less accurate six degree of freedom transducers. The poor accuracy and lack of completeness of the measurement throughout normal ranges of motion of the knee are major deficiencies.
In conducting tests, it is also of course necessary to have indications of the magnitude of the forces being exerted on the joint during the test. In order to correctly assess the extent of ligamentous damage, it is necessary to have data concerning the stiffness or stability of the joint under a great variety of external forces and moments. Accuracy of the order of 5N with ranges as high as 1,OOON may be required for such tests.
Devices presently available typically are single axis force transducers which are usually interposed between the examiner and the joint being examined. Thus, the examiner's ability to pursue his normal method of assessment, including palpitation and visual observation of measured motion, is restricted. In addition, patient apprehension, which is detrimental to well performed laxity testing, is significantly increased by devices attached directly to an injured and sensitive joint. Finally, attachments of measuring devices near the joint may affect the joint stiffness itself.